The present invention relates to a code division multiple access (hereinafter referred to as CDMA) and a method of supplementing a service area.
Various systems such as mobile telephone systems, automobile telephone systems or pager systems are provided as terrestrial mobile communication systems. A demand for such mobile communication systems is increasing at a noticeable rate and frequency assignment is becoming tighter and tighter. For this reason, in the mobile communication systems, an improvement in the efficiency of the utilization of frequencies is desired.
Several prior arts associated with the mobile communication systems will be described below. Japanese Utility Model Laid-Open No. 4644/1993 discloses a mobile communication system for realizing the next talk during congestion.
Japanese Patent Laid-Open No. 245261/1994 discloses an art which relates to a base station selecting method for a cellular wireless telephone system. In accordance with this base station selecting method, in an area covered by a general base station, an auxiliary base station is disposed for a particular region where a temporal traffic variation is large or for a singular location where channel switching continuously occurs. The auxiliary base station has only a common control wireless channel but has no wireless talk channels. A control station controls both the general base station and the auxiliary base station.
Japanese Patent Laid-Open No. 193856/1995 discloses an art which relates to a wireless base station-accommodated exchange system which is, even if communication becomes impossible in a service area of one wireless base station, capable of extending the service area of another wireless base station to eliminate a region which radio waves do not reach.
Japanese Patent Laid-Open No. 289366/1996 discloses an art which minimizes, by the use of equipment which is presently operating, the number of service areas in which communication is impossible, thereby suppressing call losses to remedy call losses which occur due to temporary concentration of traffic and to a malfunction of a wireless base station.
International Patent Publication No. 503114/1996 provides an art which relates to a channel selecting method and a base station for a mobile wireless system. In this mobile wireless system, a master base station exchanges signals with a subscriber station on a control channel which has a down link frequency for sending a signal from the base station to the subscriber station and an up link frequency for sending a signal from the subscriber station to the base station. An additional base station monitors the subscriber station at the up link frequency of the control channel of the master base station. To select the best master base station, the additional base station measures the quality of the control channel of a predetermined master station, and selects a master station whose up link frequency of the control channel is to be monitored, on the basis of the quality of the down link frequency of the control channel.
In parallel with the above-described art, an art for improving resistance to cochannel interference is proposed to improve the efficiency of the utilization of frequencies, particularly, spacial utilization efficiency. This art utilizes spread-spectrum techniques and is called a CDMA communication scheme, and has characteristics resistant to cochannel interference.
The CDMA communication scheme is a method of realizing channel separation not by means of time or frequency but by allowing each base station to use a different code (spreading code) in multiple access which enables a plurality of mobile stations (users) to divide and share a transponder or the like by wireless connections from remote locations.
Specifically, the CDMA communication scheme utilizes the fact that spread-spectrum signals are approximately orthogonal. On a transmitting side, information on individual mobile stations is spread (spreading modulation) by spreading codes unique to the respective mobile stations, and the obtained spread-spectrum signals are transmitted in the state of being multiplexed within the same frequency band. On a receiving side, information is extracted by using the spreading code of a desired mobile station (spreading demodulation). Since interference and noise which occur on the receiving side do not correlate with the spreading code, they are eliminated during spreading demodulation.
By adopting the CDMA communication scheme, it becomes possible to reuse the same frequency band at adjacent cells to make spacial utilization efficiency very high, whereby it is possible to improve the efficiency of the utilization of frequencies.
In the CDMA communication scheme, down link channels from base stations to mobile stations are set up as communication paths from one base station to multiple mobile stations, so that interference can be suppressed by making spread-spectrum signals orthogonal to each other.
However, on up link channels from mobile stations to base stations, since individual mobile stations exist at different locations and independently transmit signals, a difference in propagation delay time occur between signals from the mobile stations. For this reason, the orthogonality of codes cannot be maintained and interference is caused in the signals of mobile stations inside a cell and the signals of mobile stations outside a cell. Therefore, in the CDMA communication scheme, on the up link channels, a signal from one mobile station becomes a source of interference with communication at another mobile station.
The quality of mobile communication is determined by Eb/(No+Io), where Eb is the transmission power per bit at a receiver of a base station, No is the thermal noise power per bit at the receiver of the base station, and Io is interference power. As described above, since the interference power Io increases as the number of mobile stations increases, the transmission power Eb needs to be increased to maintain the quality of communication.
When a mobile station is performing communication at its maximum transmission power, the transmission power is impossible to increase. Therefore, if the mobile station desires to maintain the quality of communication, the mobile station needs to travel closer to a base station and reduce propagation losses to increase the Eb/(No+Io) of the receiver of the base station.
In the CDMA communication scheme, as the number of mobile stations which simultaneously perform communication increases in a cell, the radius of the cell reduces. To avoid such cell reduction, it is common practice to arrange base stations to leave a certain margin with respect to the number of base stations predictable in a certain cell area.
However, the method of arranging base stations to leave a traffic margin causes the problem that the number of approach links between an exchange and base stations increases. In addition, if the number of mobile stations suddenly increases, the resultant cell reduction causes the problem that mobile stations cannot receive mobile communication service.
An object of the present invention is to provide a CDMA mobile communication system which is capable of reducing approach links even in a cell area in which a traffic variation frequently occurs, by operating a supplementary base station only when traffic increases.
Another object of the present invention is to provide a service area supplementing method suited to the CDMA mobile communication system.
A CDMA mobile communication system according to the present invention comprises a mobile station, a base station to be connected to the mobile station by a wireless channel, at least one adjacent base station adjacent to the base station, and a base station control unit to be connected to the base station and to the at least one adjacent base station by wireless channels.
In accordance with one aspect of the present invention, at least one supplementary base station is disposed for covering an area near a boundary of a service area to be covered by the base station and an area near a boundary of a service area to be covered by the at least one adjacent base station.
The base station comprises a mobile station-directing measuring/comparing section for selectively outputting a hand-over request signal indicating that a hand-over to the at least one adjacent base station is impossible, on the basis of data indicative of a received power level of a talk signal or a packet signal from the mobile station.
The base station control unit comprises a station arrangement information table having information indicative of an arrangement of the base station and the at least one supplementary base station with respect to a service area.
The base station control unit refers to the station arrangement information table in response to the hand-over request signal, selects a supplementary base station that executes the hand-over, and outputs to the selected supplementary base station an operation start command which commands the selected supplementary base station to start an operation. When a first identification signal which is information for identifying the selected supplementary base station is outputted from the selected supplementary base station to the mobile station, the base station control unit also outputs a supplementary base station power measurement command so that the mobile station selectively outputs a supplementary base station measurement result signal based on data indicative of a received power level of the first identification signal. The base station control unit further outputs a hand-over execute command in response to the supplementary base station measurement result signal.
The at least one supplementary base station comprises a connection/disconnection control section for generating a control signal in response to the operation start command and for setting up a wireless channel to the base station control unit in response to the hand-over execute command, and a start/stop control section for outputting a start signal in response to the control signal so that the first identification signal is outputted.
A service area supplementing method according to the present invention is applied to the CDMA mobile communication system, and includes a step (a) of selectively outputting a hand-over request signal indicating that a hand-over to at least one adjacent base station is impossible, on the basis of data indicative of a received power level of a talk signal or a packet signal, a step (b) of selecting a supplementary base station that executes the hand-over, in response to the hand-over request signal, and outputting an operation start command which commands the selected supplementary base station to start an operation, and a step (c) of executing the hand-over from the base station to the selected supplementary base station on the basis of data indicative of a received power level of a first identification signal which is outputted from the selected supplementary base station as information for identifying the selected supplementary base station, in response to the operation start command.